The invention relates to a method of manufacturing a dispenser cathode, in which method tungsten and a scandium-containing material are used to form a cathode body which is also provided with a barium-containing component.
Such a method is known from European Patent Application No. 298 558, which is laid open to public inspection. In the known method, tungsten powder and a scandium-containing powder, consisting of pure scandium or scandium hydride, are mixed in a ratio of 95:5 % by weight, whereafter the powder mixture is compressed and sintered to form a cathode body of substantially porous tungsten in which the scandium is distributed in oxidized form. The cathode body is further provided with a barium-containing component by impregnating the cathode body with molten barium-calcium-aluminate at an elevated temperature.
Such a cathode is commonly referred to as mixed-matrix scandate cathode and comprises a porous matrix of predominantly the high-melting metal in which oxidized scandium (scandate) is distributed, the barium-containing component, generally in an oxidized form, being present in the pores of the matrix.
The oxidized states of scandium and barium will hereinafter be referred to as scandium oxide and barium oxide, respectively. However, unless expressly stated, they are not limited to pure stoichiometric compounds. For example, the oxidized states can contain intermediate forms of stoichiometric oxides, so-called mixed oxides. Also, if hereinafter reference is made to scandium this should not be construed as to be limited merely to pure, atomic scandium but might as well relate to some kind of scandium compound, and particularly to scandium oxide.
The barium-containing component facilitates the formation of a mono-atomic layer which contains barium at the emissive surface of the cathode. Said barium originates from the barium-containing component in the cathode body, which component is reduced to barium by the matrix metal. By virtue of the mono-atomic top layer, the work function of free electrons in the matrix is sufficiently reduced to enable electron emission. Since the mono-atomic top layer continuously looses barium as a result of the inevitable evaporation of barium, barium must be dispensed continuously to preserve the layer, which explains the name of such a cathode. Said dispensation takes place in that, during operation, barium oxide, which may be reduced already, migrates from the pores to the emissive surface where it replenishes the mono-atomic layer.
In such a mixed-matrix scandate cathode, the work function of the electrons is further reduced due to the fact that in addition to barium the mono-atomic top layer also contains scandium. As a result, such a cathode has an extremely high efficiency, enabling a relatively strong electron emission to take place at relatively low temperatures. For example, a cathode of the type mentioned in the opening paragraph enables an electron emission above 100 A/cm.sup.2 to be realised at a comparatively low operating temperature of approximately 1000.degree. C., said electron emission being more than a factor of 10 higher than that of a dispenser cathode which does not comprise scandium. Consequently, a cathode of the type mentioned in the opening paragraph is very suitable for use in an electron tube, in particular a display tube in which an image is displayed on a display screen by means of an electron beam generated by the cathode, or a pickup tube in which picture information is read from a target by means of an electron beam generated by the cathode.
However, a problem which arises when the cathode is used in the above-described way is the inevitable presence of a small quantity of residual gases in the vacuum tube. These gas molecules can be ionized by the electron beam or otherwise, so that positive ions are subsequently accelerated towards the emissive surface of the cathode by the prevailing electric fields, where they are incident on the vulnerable mono-atomic top layer. Consequently, this top-layer will soon disappear, if both barium oxide and scandium oxide are not continuously dispensed to the layer.